The hypothesis that N-nitroso compounds are carcinogenic because they are converted to direct alkylating agents by only target tissues is attractive, but most studies designed to test the hypothesis have investigated the formation of alkylated bases only after a single administration of a near lethal dose of the carcinogen. Furthermore, attempts to correlate the formation of 7-methylguanine only and carcinogenesis have not been successful. It is proposed here to examine quantitatively the nature and extent of alkylation of nucleic acids in liver, kidney, lung, pancreas and colon by 6 carcinogens which are converted to alkylating agents at widely different rates by these tissues. The sensitivities of these to the carcinogenic activity of diethylnitrosamine, N-nitrosoheptamethyleneimine, N-nitroscopyrrolidine, N-nitrosomorpholine, methylnitrosourea, and 1,2-dimethylhydrazine are already established and will be used as comparators with sites of alkylation. Adult rats, and in certain cases hamsters and guinea pigs, will be given single injections of carcinogen and isolated nucleic acids will be hydrolyzed and assayed for at least seven alkylated bases including 0-6-methylguanine by high pressure liquid chromatography. Dose-response characteristics will be determined for each alkylated base and compared to the known sensitivity of each tissue to the carcinogens. Changes in the extent to which alkylation at specific sites occurs will be determined in rats exposed continuously to low carcinogenic doses of 1,2-dimethylhydrazine, nitrosoheptamethyleneimine, diethylnitrosamine, and nitrosomorpholine during the tumor induction period. Comparisons similar to those in the acute will be made.